Scanning Tunneling Microscope (STM)
The ability to image surfaces at atomically precise levels stems from the capabilities of the STM. While there have been many different implementations since its invention in 1982, the fundamental operating principle of a STM is as follows. A conducting tip—often tungsten or platinum-iridium—that has been prepared so as to have a nanoscopic portion of the tip which allows electrons to tunnel to or from the nanoscopic portion of the tip to a sample. The tip is brought in close proximity (e.g., within a few nm) to a surface of a sample. Due to the principle of quantum electron tunneling, a current flows across the gap between tip and the sample when a bias voltage is applied between the tip and the sample. The bias voltage applied between the tip and the sample can be either polarity. If the sample is negatively biased with respect to the tip, then electrons flow from the filled electronic states on the surface into the tip. If the sample is positively biased, then electrons flow from the tip into the empty electronic states of the surface. The magnitude of the bias voltage determines the surface states that are available to tunnel into or out of. Thus, the STM provides information about the electronic states as well as the topography of the surface of the sample.
The resulting current between the tip and the sample based on the applied bias voltage varies exponentially relative to the distance between the tip and the surface of the sample. As a result of this strong dependence on the relative position of the tip to the sample, the height of the tip above the surface can be precisely controlled. Often, a piezoelectric element is used to control movement of the tip up and down (i.e., z-direction) until the measured tunnel current matches a set point value, which is in the range of 0.01-100 nA. Piezoelectric elements are also commonly used to move the tip sideways (i.e., x-y directions) across the surface of the sample. As a result, topographic images of the surface can be generated by performing a raster scan of part of the surface. Other than the feedback loop that controls the tip height, most of the tip motions are open loop and prone to creep and hysteresis.
In order to have the tip access larger areas of a sample and to allow the tip to approach a sample, many STM systems also include both fine motion control and coarse motion control for the X, Y and Z axes. For instance, coarse motion is in the range of tens of nanometers to tens of millimeters, with a precision finer than the maximum fine motion range. Most STM systems include some vibration isolation mechanisms to prevent external vibrations from disturbing the system operation.
Most STMs and virtually all commercially available STMs are designed to be laboratory tools for surface science imaging and measurements. There are many operational aspects of these instruments that make them poorly suited for computer controlled, reliable, atomically precise patterning with high productivity.
Hydrogen Depassivation Lithography
Hydrogen depassivation lithography, where a STM is used to remove hydrogen atoms from a silicon surface has been established since the mid-1990s. See, for example, Appl. Phys. Lett. 64 (15), 11 Apr. 1994, which is hereby incorporated by reference in its entirety. Further, Lyding and Hersam demonstrated that individual hydrogen atoms could be targeted and removed from a silicon substrate such as in Hersam, M. C. et al., “Silicon-based Molecular Nanotechnology,” Nanotechnology 11 (2000) 70-76, which is hereby incorporated by reference in its entirety. Since then, a number of practitioners have demonstrated the ability to form patterns using hydrogen depassivation for various purposes. However, the limitations of current hydrogen depassivation lithography techniques have prevented the use of the hydrogen depassivation lithography as viable automated manufacturing technique for atomically precise structures.
Hydrogen depassivation lithography can be accomplished at different biases and set point currents. There are several different regimes that are distinct from one another in important aspects. Typically, during depassivation lithography the sample is positive with respect to the tip so that electrons flow from the tip to the sample. At biases below 7V, the hydrogen depassivation efficiency is a strong function of the bias and is also dependent on the current. The depassivation efficiencies are in general very poor. For example according to Shen (Science Vol 268 16 Jun. 1995 p. 1591) at 3V and 1 nA more than 10 billion electrons are required to depassivate a single hydrogen atom. The depassivation efficiency rises dramatically with bias until 7V where approximately half a million electrons are required to depassivate a single hydrogen atom and there is no longer a dependence on current. From 7-12V the depassivation efficiency is essentially constant and does not depend on current.
There is also significant difference in how the electron reaches the sample from the tip at the lower biases versus higher biases. At low positive sample biases the electrons tunnel from the tip to the sample and the physics favors most electrons finding the shortest path resulting in most electrons tunneling to the same nanoscopic area with very few electrons reaching the surface outside of this very small area. Because of this process, at biases of approximately 4 volts and below, atomic precision patterning is possible. Indeed, single hydrogen atoms may be targeted and removed at low biases. Also, it is possible to create patterns where all hydrogen atoms are removed (fully saturated depassivation) within the desired area and no atoms removed outside the pattern or at most the edge of the pattern will deviate by no more than the distance of one atom from the designed pattern. This is generally referred to as atomically precise patterning in the present disclosure. At higher biases, there is the opportunity for field emission (Fowler Nordheim tunneling) of electrons from the tip into the vacuum and then a short trip along field lines to the sample. This electron path along with the fact that the tip will generally be further from the sample at higher biases results in electrons reaching the sample over larger areas. The result is that atomic precision patterning is not possible.
While hydrogen depassivation has been reported with a negative sample bias, the efficiency of the process is even worse than that of a positive sample bias and the mechanism of depassivation in this context is not well understood. On the other hand, the extremely low depassivation efficiency at low negative sample biases permits effective imaging without imparting depassivation. It is also possible to image the sample with little or no depassivation at low positive sample biases with low set point currents.
Hydrogen depassivation lithography is the only lithographic process that has demonstrated atomic resolution patterning and it has the advantage of having the same tool that does the patterning, the STM, to also be capable of examining the patterning area before and after exposure. However, in the atomically precise mode of patterning the process is extremely inefficient.
Electron Beam Lithography
In some respects, depassivation lithography is effectively e-beam lithography operating on the limit of a thin resist. It can be instructive to compare this subset of e-beam lithography with conventional e-beam lithography. Conventional e-beam lithography is an important industrial and research tool that can make very fine patterns by focusing a beam of electrons down to a small Gaussian spot (or other shape) on a substrate. Electron beam columns use high voltage to generate very energetic beams, operate in high to ultra-high vacuums, and use magnetic and/or electrostatic lenses, blankers, and deflectors to shape, blank and scan the beam. The electron beam exposes a thin layer of resist material (usually a polymeric material) and changes its properties in a manner (usually chemical bond breaking or crosslinking) so that a portion of the resist (either the exposed or unexposed portion) can be removed by a subsequent development step. The pattern formed from the resist is transferred to the underlying substrate by a variety of processes that can be additive (patterned deposition) or subtractive (patterned etching). A combination of electron beam deflection and blanking (and often the movement of the substrate) allows a designed pattern to be exposed over the desired portion of the substrate. The main industrial use of e-beam lithography is to write masks used in the production of very large scale integrated circuits.
While hydrogen depassivation lithography is also a form of e-beam lithography in that it uses beam of electrons with a small spot size to serially expose a resist to create a desired pattern, there are a number of significant differences. The electron beam energy is typically 1,000-100,000V for conventional e-beam lithography and is 100V or less for hydrogen depassivation lithography. Principally because of this difference in electron energy, with conventional e-beam lithography, there is significant proximity exposure of the resist, while there is little if any proximity effect exposure with hydrogen depassivation lithography. Conventional e-beam lithography systems have complicated and expensive electron optics while hydrogen depassivation lithography has a tip and a substrate. Conventional e-beam lithography systems cannot image without exposing the resist, while this is possible with hydrogen depassivation lithography. In conventional e-beam lithography, higher electron energy leads to smaller optimized spots but lower efficiency for exposing resist. In hydrogen depassivation lithography a higher current at a fixed electron energy will bring the tip closer to the sample which can produce a smaller exposing spot. In hydrogen depassivation lithography, higher electron energy leads to larger spots and higher resist exposing efficiency. In conventional e-beam lithography, for optimized beams at a fixed electron energy a smaller spot will require a smaller beam current.
While it is conceivable to adapt some of the techniques used in conventional e-beam lithography systems to develop a robust reliable STM based e-beam lithography system, the significant differences outlined above will require novel approaches in that development.
Accordingly, there remains a need for improved methods, devices, and systems for forming atomically precise structures.